The invention relates generally to high-power, mid-IR optically pumped unstable resonator lasers and in particular to a means to produce a high-power output while maintaining a near diffraction-limited beam.
High power optically pumped mid-IR lasers have been demonstrated with peak powers in the 5 to 15 W range, CW powers in excess of 2 W, and lasing wavelengths in the 2 to 10 micron region. However, these broad area lasers typically display degraded beam quality as the pump intensity is increased; they may go from 2-3 times diffraction limited near threshold to 8-15 times diffraction-limited at ˜30 times threshold. This limits the devices applicability since it becomes difficult to couple the radiation into a small numerical aperture fiber or to focus the radiation in the far field. There are a large number of applications that would directly benefit from high power mid-IR output in a nearly diffraction limited beam. These applications include infrared countermeasures, free-space optical communication, remote sensing, laser marking, and various medical applications.
A number of approaches have been utilized in near-IR semiconductor diode lasers (electrically pumped lasers) to achieve high-power operation with diffraction limited output. These include tapered amplifiers, angled injection into traveling wave or reflective wave amplifiers, coupled narrow stripe lasers, and various unstable resonator (UR) geometries. The UR laser concept is best understood by comparing it to the conventional Fabry-Perot (FP) laser. The conventional semiconductor laser uses an FP cavity defined by two parallel mirrors. The lasing mode undergoes multiple reflections at the cavity mirrors and the mode is directly counter-propagated. In contrast, the UR laser is characterized by counter-propagating diverging cylindrical waves diverging from fixed virtual source points. By avoiding direct counter-propagation UR's suppress filamentation and maintain excellent beam quality with all the radiation diverging from fixed high-brightness virtual source points. Consequently, the UR laser is a high brightness source since near diffraction-limited beam quality can be preserved even with broad laser cavities and under conditions of high current injection or optical pumping.
In one approach to the UR design for electrically injected semiconductor lasers, as described in U.S. Pat. No. 5,179,568, the UR cavity was formed with curved facets for the end mirrors. However, a high precision manufacturing process, such as ion-beam milling, was required to maintain losses at the mirrors within acceptable limits. The required high-precision does not permit such devices to be formed in quantity and with reasonable reproducibility.
In another approach a train of weak negative lenses is grown into the structure and causes the main mode to expand laterally as it propagates. However, for large changes in the index of refraction each lens will act as a reflecting surface and will introduce further losses in the cavity. (Paxton et. al. “Semiconductor Laser with Regrown-Lens-Train Unstable Resonator: Theory and Design,” 29, IEEE J. Quant. Electron. No. 11, pp 2784-2792, 1993)
In another approach to the UR design for electrically injected semiconductor lasers, as described in U.S. Pat. No. 5,438,585 an anti-guide region is optically coupled to the active region of the laser. The anti-guide region has a lateral variation in the effective index of refraction that forms a diverging medium that causes higher order modes to experience higher losses in the resonant cavity. However, the main mode suffers higher losses as well and the growth of this structure requires multiple epitaxial growth steps thereby complicating the fabrication process.